1. Field of the Invention
The present invention relates to vehicle control systems, and more particularly to a vehicle control system in which learned value data is stored into an electrically rewritable nonvolatile ROM such as an EEPROM.
2. Description of Related Art
Conventionally, in a vehicle control system for controlling a vehicle engine or automatic transmission, a learning control method is in widespread use for correcting control parameters and logic data through evaluation of past control results to eliminate an adverse effect due to deterioration with age or individual difference of a controlled object. Learned value data including control parameters attained by means of learning control is held in a standby RAM (backup RAM which is always supplied with battery power).
However, if a battery is disconnected from a vehicle intentionally or accidentally, learned value data held in the standby RAM will be lost. Therefore, in recent years, learned value data has been stored into an electrically rewritable nonvolatile ROM such as an EEPROM. On occurrence of a battery disconnection, the learned value data is transferred from the nonvolatile ROM to an ordinary RAM. In this manner, previously attained learned value data is used continuously.
In the abovementioned arrangement, by increasing a frequency of updating learned value data in the electrically rewritable nonvolatile ROM(EEPROM), the latest possible learned value data can be used for control operation in the event of a battery disconnection. Under the present-day technological constraints, however, this kind of nonvolatile ROM has a guaranteed upper limit of rewritable count, which is specified by each semiconductor device manufacturer. If it is attempted to rewrite data thereinto beyond the guaranteed upper limit of rewritable count, the data may not be stored accurately therein. Therefore, it is not desirable to frequently update learned value data in the electrically rewritable nonvolatile ROM.
In Japanese Unexamined Patent Publication No. 79379/1993, for example, there is disclosed a vehicle control system arranged for writing the latest learned value data into an EEPROM at intervals of a certain period of time. In consideration of a useful lifetime of an automobile (up to 15-20 years), the possibility of exceeding a specific guaranteed upper limit of rewritable count of the EEPROM is very high. Furthermore, in this kind of vehicle control system in which operating power is supplied when an engine ignition switch of a vehicle is turned on, the vehicle is not always run while the vehicle control system is activated by turning on the ignition switch. Consequently, useless learned value data may be written into the EEPROM at the time of updating, thereby causing a count of writing data into the EEPROM to increase more than necessary.